1. Field of the Invention
This invention relates to an improved method for the manufacture of cast articles from Al-Si-Cu type or Al-Si-Cu-Mg type casting alloys.
The term "Al-Si-Cu type casting alloys" as used herein means the Al-Si-Cu alloys of the type corresponding to those designated as AC2A, AC2B and AC4B by Japanese Industrial Standard for Aluminum Alloy Castings (JIS H5202) and the term "Al-Si-Cu-Mg type casting alloys" as used herein means the Al-Si-Cu-Mg alloys of the type corresponding to those designated as AC4D, AC8A, AC8B and AC8C by JIS H5202.
Since these Al-Si-Cu type and Al-Si-Cu-Mg type casting alloys have good casting properties, possess appropriate strength and toughness and excel in resistance to thermal shock, they are widely used for casting parts for engines in automobiles and ships and other machine parts.
2. Description of the Prior Art
Heretofore in the manufacture of cast articles of the alloys in question, the alloy cast bodies obtained by casting the molten alloy material into molds had to be subjected to a solution heat treatment and then to quenching and an artificial aging heat treatment in order to acquire the necessary strength and toughness. This solid solution heat treatment has to be carried out at elevated temperatures within the range of from 500.degree. to 530.degree. C. for long periods ranging from several hours to ten or more hours. Further, the treatment for artificial aging has to be continued at temperatures within the range of from 140.degree. to 230.degree. C. for similarly long periods ranging from several hours to ten or more hours. These treatments have necessarily affected the casting operation adversely notably from the standpoints of production efficiency and heat economy.
With a view to avoiding these disadvantages involved in the casting of the type requiring thermal treatment, there has been proposed a method which, as disclosed in Japanese Patent Publication No. 10901/1962, for example, comprises quenching the cast body to room temperature immediately after casting, thereby simultaneously effecting quenching and formation of supersaturated solid solutions and subsequently subjecting said cast body to treatment for artificial again.
When the latter method is applied to the alloys of the present invention, since the Cu component, among other components of the alloys, cannot be made to form a thorough solid solution, the cast articles fail to acquire sufficient strength from the thermal treatment. Because of the absence of a uniform solid solution, the cast articles suffer from notable variation in strength distribution, sustain serious deformation and offer insufficient resistance to thermal shock.